The present invention relates to an improved process and apparatus for the production of organometallic compounds, in particular organo compounds of Group 3a metals, especially trimethylgallium.
Organometallic compounds such as trimethylgallium are commonly employed as a gallium source in the fields of epitaxial semi-conductor growth and/or processing, vapour or plasma etching, plasma deposition or thin film deposition, for example metalorganic chemical vapour deposition (MOCVD). The compounds are generally prepared using a batch process wherein the reactants are fed into a reaction vessel and the product is collected once the reaction is complete. Trimethylgallium is normally prepared by means of such a batch process using gallium trichloride and a suitable alkylating agent such as a Grignard reagent, Lithium reagent or trialkylaluminium. This process provides compounds that are sufficiently pure but requires the use of large reaction vessels to produce a sufficient yield of product. The production rate of the compound is also limited due to the necessity to charge the reaction vessel, add specified amounts of the reactants, allow the reaction to take place and then collect the product and clean the vessel before commencing the process again. Cleaning the equipment between runs of the reaction may also lead to increased contamination of the product.
GB-A-2 123 423 (The Secretary of State for Defence) relates to the preparation of trialkylgallium compounds using a dialkylether. However, such a reagent can result in the product containing oxygen impurities. DE 11 58 977 (Siemens-Sciuckertwerke AG) also describes a process for the preparation of trialkylgallium compounds, whereby one equivalent of gallium trichloride is added to three equivalents of an aluminium trialkyl in solution in a hydrocarbon solvent or an ether. The process is carried out as a batch process to enable sufficiently pure product to be obtained. Similarly, Inorg. Synth (1974) 15, 203-7 and Chemical Abstracts, vol. 81, no. 17, 28 October 1974 (1974-10-28) and TR. Khim, Khim. Tekhnol (1973), (4), 40-41 relate to the preparation of trialkyl metal compounds using batch processes.
It is an aim of the present invention to provide an improved process and apparatus for the preparation of organometallic compounds, in particular the production of trimethylgallium, which overcomes the above mentioned drawbacks.
Accordingly, a first aspect of the present invention provides a continuous process for the preparation of an organometallic compound comprising the steps of delivering a metal precursor and an alkylating agent separately to a reaction centre, allowing the precursor to react with the alkylating agent, maintaining the reaction centre at a predetermined temperature to cause vapourisation of the organometallic compound and collecting the organometallic compound.
A second aspect of the present invention provides an apparatus for the preparation of an organometallic compound, the apparatus comprising at least two delivery conduits and a distillation column having a reaction centre and an outlet wherein each conduit transports a metal precursor and an alkylating agent respectively to the reaction centre to allow the reactants to react together, the temperature of the apparatus being maintained such as to allow collection of the vapourised organometallic compound from the outlet thereby enabling continuous production of the organometallic compound.
The process of the present invention may be used to prepare any organometallic compound wherein the product is a volatile liquid and the by-products are less volatile.
The continuous process for the preparation of organometallic compounds requires the reactants to be fed separately into a reaction section which is preferably provided in the centre of the distillation column. Preferably the outlet is provided at or near the top of the column for collection of the product and a further outlet is provided at or near the base of the column for removal of any waste products. The equipment is maintained at specified temperatures to control the rate of removal of the product from the top of the reaction column, with the non-vapourised waste products being removed from the base of the column. The continuous delivery of the reactants in specified amounts and at a predetermined rate into the reaction section, together with the maintenance of a specified temperature/pressure differential in the column enables a continuous production of the organometallic compound to be achieved. The rate of addition of the reactants may be controlled by means of appropriate flow controllers.
The rate of removal of the product from the top of the column should be carefully controlled to avoid a build up of the product in the column and a consequence loss of the product from the base of the column with the waste. Suitable conventional instrumentation systems such as a flow controller and temperature/pressure differential, may be used to maintain the required rate of removal of the product.
The continuous process relies on an equilibrium being set up in the column such that the product goes up the column and the waste products move down for their removal at the base, for example by means of a simple overflow leg. A number of factors effect the maintenance of the equilibrium, such as the temperature of the column, the addition rates of the reactants and the take off rate of the product. The conditions employed will vary according to the reactants and the organometallic compound being produced. The temperature is preferably fixed and the main control for a set of addition rates will then be the take off rate of the product.
Preferably, the distillation column for use in the present invention is fitted with a boiler at the base and a condenser at the top, the design of which will be dependent on the heat of the reaction to be carried out in the column. Importantly, means should be provided to allow the control of the reflux ratio applied to the top of the column and to remove the heat produced during the reaction from the reaction zone.
Any suitable distillation column may be used for carrying out the process of the present invention such as packed or plate distillation columns. The column should be equipped with sufficient plates in a stripping section to ensure removal of all the product from the stream leaving the base of the column and sufficient plates in the rectifying section to ensure the pure product leaves the top of the column.
Preferably, the metal precursor is added in a suitable solvent, such as toluene. It is preferable to prime the reaction centre with the solvent prior to commencing the reaction. Preferably, the solvent has a lower volatility than the organometallic compound to allow the pure product to be collected from the top of the column while the solvent leaves the base of the column with the by-products.
The reaction centre is preferably heated prior to commencing delivery of the reactants to the reaction centre in order to achieve a steady reflux. Preferably alkylating agent is added to the reaction centre prior to the metal precursor to ensure there is an excess amount of agent present.
Collection of the product is preferably commenced once a constant temperature corresponding to the boiling point of the product is recorded near to the top of the column, preferably around 2-5 cm from the top of the column. Preferably, collection of the product is halted if a rise in temperature is recorded and re-commended once the temperature drops back to the boiling point of the product.
The process of the present invention is particularly suitable for the preparation of Trimethylgallium in a continuous fashion by the reaction between trimethylaluminium (TMA) and gallium trichloride in a suitable solvent. The reaction is extremely exothermic and virtually instantaneous and it is desirable to move heat as quickly as possible from the reaction zone in order to prevent unwanted side reactions.
The reaction centre is preferably heated to a temperature between 120xc2x0 C. to 140xc2x0 C, more preferably 130xc2x0 C. until a steady reflux is achieved. The reactants may then be added, preferably commencing with the TMA. The ratio of active ingredients in the column should be such that there is an excess of TMA at all times to ensure full conversion of the gallium trichloride to trimethylgallium. A ratio of 5:1 TMA to gallium trichloride is preferred but other ratios may be used, such as 4:1 or 3.5:1.
The TMA and GaCl3 may be added at any predetermined rate depending upon the production rate of product required. For example, the TMA and GaCl3 may be added at a rate of 90 g/hr and 75 g/hr respectively to give a production rate of 30 g/hr. Alternatively, addition rates of 250 g/hr and 315 g/hr may be used to provide a production rate of 115 g/hr.
Collection of the trimethylgallium is commenced once a steady temperature of 56xc2x0 C. is maintained near the top of the column.
The solvent is preferably one which is less volatile than the trimethylgallium, such as toluene to ensure that pure product is collected as a vapour.